In chronic renal disease, the portions of the kidney which are relatively undamaged are responsible for maintaining the function of the organ; yet deterioration often ensues, even in the absence of evidence of activity of the primary disease. Similarly, in experimental renal failure induced by subtotal nephrectomy in the rat, there is initially good compensation; later there is progressive deterioration with hypertension, azotemia, and apparently abnormal glomerular permeability with increasing proteinemia. It is the objective of this study to obtain ultrastructural data concerning the change in the glomerular filter accompanying progressive renal failure in this system. The characteristics of the filtration barrier during adaptation to subtotal nephrectomy and subsequent decompensation will be studied by electron microscopy, using stereological methods and appropriate molecular probe techniques.